Intel Mesa Driver Finally Supports Threaded OpenGL

Based off the Gallium3D "mesa_glthread" work for threaded OpenGL that can provide a measurable win in some scenarios, the Intel i965 Mesa driver has implemented this support now too.

Following the work squared away last year led in the RadeonSI driver, the Intel i965 OpenGL driver supports threaded OpenGL when the mesa_glthread=true environment variable is set.

Among the measured benefits so far to using Intel threaded OpenGL is around a 30% improvement for the Citra emulator while the Dolphin game emulator improved by 2.8x in one game. Like the Gallium3D code, the Intel threaded GL implementation also depends upon per-game white-listing within the DRIRC file.

The patch for now can be found on mesa-dev but will hopefully appear in Mesa 18.2-dev Git shortly. Benchmarks forthcoming.